IEEE 802.15.4 MAC utilizes Beacon Order (BO) and Superframe Order (SO) to implement low duty cycle transmission of nodes within an ad-hoc communication system. The ZigBee Alliance™ system protocol requires all nodes in the same network to define the same BO and SO values. As a result, all nodes have the same duty cycles.
ZigBee networks support both tree and mesh topologies. In the tree topology, the personal area network coordinator (PANC) node serves as the tree root and is the sink for data collected in the tree. In applications where other tree devices need to operate in a low duty cycle, the PANC node also applies a low duty cycle to meet the ZigBee's requirements.
However, in a practical sensor network implementation, it is not necessary for the PANC node to operate in a low duty cycle, because it is usually attached to a powered information center (like a gateway that connects to internet or database). Simulation results also show that a low duty cycle PANC also causes traffic congestion when the network size below the PANC reaches a certain limit. Since it is more practical to implement a hard-powered PANC, making a PANC node as a full duty cycle tree root can help reduce the traffic congestion problem when high network scalability is required.
However, making a PANC a full duty cycle node should not change the low duty cycle operation of other tree nodes, especially the nodes as direct children of the PANC. In a normal operation, nodes that are direct children of the PANC wake up for one sub-superframe slot to talk to the PANC or receive messages from the PANC. When PANC operates in a full duty cycle, such nodes should not wake up randomly to send data to the PANC or listen continuously to the PANC for data targeted at it. They should continue waking up for one sub-superframe slot to talk to the PANC or receive message from PANC as previously designed.
In addition, the ZigBee network specification utilizes an address scheme called Cskip for tree networks. In Cskip, a maximum number of children (CM) value is defined consistently over the same network. This means that each node can have at most CM child nodes existing under it. This is also required for the PANC. However, when a fully duty cycle PANC is operational, the number of the PANC's direct children nodes should be extended and not be limited by CM. Therefore a need exists for node scheduling and address assignment within an ad-hoc communication system that allows for a PANC node to operate as a full duty cycle tree root.
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